Abstract

Extensive efforts have been directed in the last several decades towards improving thermodynamic efficiency of industrial gas turbines for power generation plants. The central theme of the efforts is to increase the turbine operating temperature and, thus, allowing higher efficiency. Thermal barrier coatings (TBC) constitute an advanced technology to protect the metallic surface from high temperature exposure for long time operation. The TBCs protect the gas turbine components from high temperature and allows further increase in engine operating temperature which subsequently increases the efficiency of the gas turbine power plant. However, the current TBC materials are capable of allowing the surface temperature tolerance to ∼1200 °C. The present work was performed to investigate the novel hafnia based TBCs to ensure the higher surface temperature tolerance for long time operation in advanced gas turbine technology. Attention has been paid towards developing a fundamental, deeper understanding of the growth behavior, microstructure, surface-interface structure and stability, thermo-chemical and thermo-mechanical properties and durability of hafnia based TBCs. Yttria stabilized hafnia (YSH) and materials engineered from variable contents of hafnia (HfO2) and zirconia (ZrO2) stabilized by yttria (YSHZ) were investigated. The YSH and YSHZ coatings were produced by sputter-deposition onto various substrates. The deposition was made by varying the growth temperature from room-temperature (RT) to 500 °C. The crystal structure, surface morphology, thermal stability, thermal conductivity and mechanical properties of the grown coatings were evaluated as a function of the coatings' composition and growth temperature. The crystal structure analysis performed by X-ray diffraction (XRD) indicates the stabilization of hafnia-cubic phase in all the coatings. The morphology of the coatings is characterized by the columnar growth coupled with a dense structure as revealed by the scanning electron microscopy (SEM). Thermal stability evaluation performed using high temperature XRD coupled with SEM indicates the stability of these coatings to 1300 °C. Thermal measurements using photo-acoustic technique and time-domain thermo-reflectance (TDTR) method indicate an effective reduction in thermal conductivity of YSH coatings compared to pure hafnia and yttria stabilized zirconia (YSZ). Mechanical properties studied using XRD indicate a very high level of compressive residual stress within the coating. The durability test using a laboratory scale combustor rig demonstrates enhanced stability of the coating in real hot gas environment. YSH coatings were also grown by electron beam physical vapor deposition and tested to compare with coatings grown by magnetron sputtering.^